Preparation of alkali metal salts of glucuronic acid from glucuronolactone



Patented June 15, 1954 UNITED STS KALI METAL SALTS OF PREPARATION OF AL GLUCUR-ONIC ALI LACTONE William Hach, Oak Park min, Maywood, IlL, Refining Company,

No Drawing. Application October D FROM GLUCURONO- and Donald G. Benjaassignors to Corn Products I New York, N. Y., a corporation of New Jersey Serial No. 253,982

4 Claims.

This invention relates to the preparation of pure salts of glucuronic acid, particularly the alkali metal salts. These salts have been diificult of access heretofore, due to lack of a source of glucuronic acid. Quite recently glucuronic acid has become available, not as free glucuronic acid but as glucuronolactone, no practical method having been found for obtaining pure glucuronic acid directly. Theoretically, all that is necessary in order to obtain salts of glucuronic acid from glucuronolactone is to hydrolyze the lactone ring therein and neutralize the resulting glucuronic acid with a base containing the desired cation. Since hydrolysis and neutralization occur simultaneously when hydrolysis is carried out in the presence of a base, the reaction may be written as follows:

(IIHO (IJHO onon anon onon CHOH +1101; +1110 onou CIlHOH neon OHOH coon M Glucuronolactone, however, is subject to attack by alkali inat least two ways: first, contact with alkali causes rapid degradation and color formation similar to that which occurs with glucose as a result of attack on the aldehyde function, and second, contact with alkali also causes hydrolysis of the lactone ring to form the salt. The latter is the desired reaction in making salts of glucuronic acid but because of the former reaction it has been difilcult to obtain good yields of pure alkali metal salts of glucuronic acid from glucuronolactone.

It would appear obvious to those ski led in the art that degradation resulting from attack on the aldehyde function could only be avoided by adding the base used for hydrolysis no faster than it is consumed in the neutralization reaction, so thathigh pH levels cannot develop. However, hydrolysis of gluouronolactone proceeds very slowly at pH levels below neutrality and when, for example, potassium hydroxide is added to a solution of glucuronolactone the pH level rises above '7 almost immediately. In order to cause the reaction to proceed at a reasonably practical rate, alkali must be added at a rate such that a pH level somewhat above 7 is maintained in the system. Even if the alkali is added sufiiciently slowly that the pH level never rises above about 8, the reaction proceeds so slowly that by the time the required amount of alkali has been added the solution has become quite dark in color and a substantial amount of glucuronic acid has been destroyed. When such high color is developed, it cannot be removed efilciently with activated carbon and yields of satisfactory quality product from such liquors are disappointingly low. Carrying out the reaction at somewhat elevated temperature allows the operation to be completed in shorter time, but rate of alkaline degradation also increases with increasing temperature so that little ad vantage is gained.

Glucuronic acid itself will react readily with a base to form a salt, and alkaline loll levels do not develop in the system until substantially all the required amount of alkali has been added. Therefore, solutions of pure glucuronic acid can be neutralized with bases in the customary manner to produce good yields of the corresponding salts. As already stated, however, known processes for obtaining glucur'onio acid result in glucuronolactone rather than glucurcnic acid, or at least in mixtures containing relatively large amounts of the lactone. Consequently, no matter how carefuly available materials are treated with alkaline reagents in the manner customarily used to prepare salts, prolonged reaction times are required, alkaline pl-l levels cannot be avoided, and colored products and low yields result.

Aqueous solutions of glucuronolactone tend to revert to equilibrium mixtures of glucuronolactone and glucuronic acid on standing. At ordinary temperatures several weeks are required for equilibrium to be established and the mixture then contains to per cent free glucuronic acid. Such solutions will consume rapidly #30 to 70 per cent of the required alkali to makethe corresponding salt, without developing alkaline pH levels but, when alkali addition is continued to convert the remaining lactone to salt, essentially the same difliculties arise (i. slow reaction and color formation) as if the solution had not been allowed to equilibrate. In any event the time required to permit establishment of the initial equilibrium makes such a procedure impractical.

The time required for solutions of glucurono lactone to become equilibrated can be shortened substantially by increasing the temperature; but glucuronolactone solutions equilibrated at higher temperatures contain decreasing amounts of free glucuronic acid as temperature is increased. Thus, if glucuronolactone is dissolved in water and the solution is heated at 10G C. equilibrium is established within a few hours, but the solution then contains only about 30 per cent free acid, so no advantageis gained. Mineral acids act as catalysts in hastening establishment of euuilib rium in solutions of glucuronolactone, without affecting the composition of the equilibrium mix ture as far as glucuronolactone and glucuronic acid are concerned. However, solutions allowed to equilibrate in this way exhibit the same behavior during neutralization as when no mineral acid is employed and, in addition, the presence of mineral acid gives rise to inorganic salt upon neutralization and this results in contaminated products.

We have found that little improvement is realized from the standpoint of color formation or yield by employing milder alkalies, such as sodium or potassium bicarbonate, in place of the corresponding hydroxides for neutralizing solutions containing glucuronolactone. Thus, none of the conventional methods for preparing salts from acids or their lactones produces high yields of a satisfactory product when applied to solutions of glucuronolactone.

It is an object of the present invention to provide a new and improved method for preparing pure salts of glucuronic acid. It is a further object to provide a method of preparing pure alkali metal salts of glucuronic acid. A further object is to provide a method of preparing pure salts of glucuronic acid in high yield from glucuronolactone. Other objects will appear hereinafter.

Our invention, in general, comprises rapidly and intimately contacting essentially chemically equivalent amounts of glucuronolactone and the appropriate alkali metal hydroxide under controlled conditions of temperature and preferably controlled conditions of concentration to permit most economic recovery, to produce salts of glucuronic acid.

Our invention lies in our discovery that, in spite of the fact that glucuronolactone hydrolyzes very slowly at pH levels below neutrality whereas it is destroyed rapidly at pH levels above neutrality, nevertheless high yields of pure alkali metal salts of glucuronic acid can be prepared from glucuronolactone and the appropriate alkali metal hydroxide when these are allowed to react quickly under controlled conditions.

. While order of addition of the reactant bases and acids usually is unimportant in reactions leading to salt formation, in this particular case it would appear obvious to those skilled in the art that addition of a solution of glucuronolactone to one containing the required amount of alkali, although resulting in hydrolysis of the lactone and neutralization of the resulting glucuronic acid, would cause inordinate destruction of the aldehydic compound because of the large excess of alkali present in the initial stages of the reaction. Quite unexpectedly, however, we discovered that when dry glucuronolactone was added rapidly and with agitation to a solution containing the theoretically required amount of sodium or potassium hydroxide exceptionally high yields of pure sodium or potassium glucuronate could be recovered from the reaction mixtures. Equally satisfactory results were obtained when glucuronolactone was dissolved in water and added to a solution containing the theoretically required amount of sodium or potassium hydroxide, or the solutions could be mixed in reverse order, provided in all cases the reactants were brought together very rapidly. Thus, unexpectedly, we discovered that the time allowed for reaction was a far more eifective variable than concentration of reactants, reacr should be as short as possible.

tion temperature, or even pH level, when the latter variables were controlled within reasonable limits, in affecting the yield and quality of product attainable. Concentration of reactants and reaction temperatures are not critical and relatively high pH levels can be tolerated, provided the period of time the mixture remains on the alkaline side of neutrality is minimal. Although high pH levels do result temporarily when equivalent amounts of the reactants are mixed together quickly, the hydrolysis and neutralization reactions proceed rapidly under these conditions even at low temperatures. But particularly if the amount of glucuronolactone employed represents a slight excess over the alkali, the pH of the mixture drops quickly to neutrality or below as a result of the neutralization so that little alkaline degradation or color formation occurs under these conditions unless excessively high temperatures are employed or allowed to develop. Unduly high temperatures favor these adverse reactions and we have found that temperatures in excess of 70 to should be avoided during the reaction itself for best results. This places some limitations on practical concentrations of reactants since all neutralization reactions are exothermic and temperature control becomes difficult if reactants are employed in highly concentrated form as will be readily apparent to those familiar with the art.

The time required for mixing the glucuronolactone and alkali metal hydroxide is the most critical factor in carrying out our invention and The time should preferably not exceed 5 minutes, but it may be as long as 10 minutes. Longer times result in increased color formation, greater destruction of glucuronic acid and, consequently in smaller yields and colored products.

The system should be subjected to eflicient agitation during the mixing operation so that intimate contact of the reactants is maintained.

The preferred temperature for carrying out the reaction is about '20 to 90 F. but temperatures as high as l10-130 F. may be used.

No more than the chemically equivalent amount of alkali metal hydroxide should be added. Preferably the ratio of alkali metal hydroxide to glucuronolactone should be 0.9 mole of the former to 1 mole of the latter.

We have discovered that further advantage leading to improved yields of high quality product may be realized by choosing concentrations of reactants such that a large portion of the salt formed will crystallize after the reaction is complete without concentrating the reaction mixture, and that the amount recovered in this manner may be further increased if a suitable amount of methanol or some other organic solvent is incorporated in the reaction system to decrease the solubility of the salt therein; aqueous methanol may be employed as solvent for either or both of the reactants, or methanol may be added to the reaction mixture following combination of the aqueous reactants. Since the crude glucuronolactone usually employed as one of the reactants contains some color and some color develops during the reaction in any case, carbon treatment is beneficial. We, therefore, prefer to bring the aqueous reactants together and then add carbon, hold the mixture for a sufiicient time to accomplish decolorization at a temperature such that the salt will not crystallize, then remove the carbon and add the proper amount of that col accuses methanol and allow the salt to crystallize. The carbon treatment may be carried out attemperatures as high as 130 F. since the alkali metal hydroxide has been consumed in the reaction and the efl'ectof high temperatures is much less at neutral or slightly acidic pH levels.

The alkali metal salts of glucuronie acid prepared by the process of our invention are essentially white and do not require recrystallization. It is essential for highest yieldandpure.

white product that .the reaction and isolation operations be carried out as rapidly as possible. If dilute solutions containing the proper amounts of reactants are; brought together rapidly little color develops during the reaction, but the time 1' required to concentrate the resulting mixture to a point where the salt. willcrystallize is detrimental from the standpoint of color development unless temperature. during. the. concentration -operation is inaintainedat levels below those eco-' noinical for. commercial operation. Carrying. out the reaction in a mediumsuch that amaximurn firstcrop of. rystals is produced therefore has particular advantagemin. our. process. While additional crops. can berecovered fromumother liquorsgthis.involves evaporation and accompanying color-development, so that the greater --the-first.crp .yieldthe higher willbe thafinal e .rotheir handythe final reaction mixture should not. be so concentrated that? the salt will crystallize erall recovery. of high quality product. On the before decal-arising carbon can berernoved, or orcarriesdown with the crystals. Pure. potassium ornpure sodiumaglucuronate can be prepared from glucuronolactone in yields of 85 to 95 per cent by our process. The former crystallizes as the dihydrate and the latter as the monolaydrate. Preferred procedures forpreparing the two salts difierslightly because of their differing solubilites, sodium glucuronate being. substantially more. soluble than potassium glucuronate.

In order to attain the abovementioned advantage of maximum first crop yield without the .ecessity of concentrating the reaction mixture,

the amount of water for 0.9 mole of alkali metal hydroxide and 1 mole of glucuronolactone should be. within. the range of to 75., and preferably 55, moles when potassium hydroxide is the alkali metal hydroxide, and within the range of 25 to"- 50, and preferably 35, when sodium hydroxide is the alkali metal hydroxide.

amount of methanol used in recovering the potassium salt should be about 20 to 40, and

preferably 30,per cent on a volume basis of the solution obtainedfrom the reaetionand for recovering the sodium salt 45, per cent.

Ammonium glucuronate cannot beprepared by our process, probably due to the Well known fact that ammonia reacts with carbohydrates containing aldehyde groups to form compounds of to 55, .and. preferably the. glucosylamine type. The glucuronolactone employed in our process does not need to be spe cially purified; crude glucuronolactone is entirely satisfactory.

instantaneously into one liter of cool F.)

lNlpotassiuin hydroxide (1,",11'1016) containing 2 ,g. ofpactivated. carbonandstirred rapidly until -tl1e.-pH,level:dropped below about 8.5. The reaetlonmixture was then warmed to F. and .held at that temperature for 30 minutes with constant agitation. .It was then filtered, car" bon cake was washed with 50 ml; water, and

.dnl.L ofirnethanolywasadded to the combined fil- $1 7336 land wash.,1ntter 16 hours at room temperature it thecrystalline potassium glucuronate dihydrate which had formed Washed with 190 ml. of

filtered off and ec/eo methanol/water :solution. The dried crystals weighed 209 g.

; a rate such that the mixture was never :endof the neutralization 4 g. of activated carbon pure white sodium .glucuronolaotone was mixture wasstirred rapidly until the pH level dropped below about 8.5, then warmed to 129" F. agitated for 30 minutes, filtered, the carbon cake rinsed with 50 ml. water, and 650 methanol was added to the combined filtrate and rinse. After-l6 hours a crop of crystalline sodium glu- 1curonate .monohydrate .wasremoved by, filtration andwashed with 190 ml. of .80/20 methanol/water solution. The dried crystals weighed 179 g.

.The combined mother liquor and wash was concentrated to .25". Baum under reduced pres- ...sure..and ml. :16 hours a second ;,by filtration and methanol was added. After crop ,of crystals was isolated washed with 90 ml. of 80/20 methanol/water solution. The overall yield of glucuronate monohydrate was thus 200 g., or 86. percent calculated on basis that only one. mole of alkali had been used in the reaction.

EXAMPLE III Preparation of potassium glucoronate by titration of glucuronolactone solution with alkali at less than 7.0 pH

wIn thisexperiment 176 g. of recrystallized dissolved in 500 m1.v of water at F. and 2 N potassium hydroxide was added dropwise with constant agitation at pH value of the reaction greater than 7.0. Toward the Wasadded. Considerable color developed dur ingthe neutralization which required 2 hours.

centp The third crop was 1 The following examples, which are intended as informative and typical and not in a limiting sense, will further illustrate the invention.

EXAMPLE I Preparation of potassium glucuronate dihydrate One. hundred and .ninety grams ..(.l.08..moles.)1. of crude crystalline g-lucuronolactone-was dumped Three successive. crops of potassium glucuronate dihydrate were isolated by concentration of the filtered product for an overall yield of 82 per oil color, however, andyield of acceptable potassium glucuronate di'nydrate was only 73 per cent. This yield does not compare favorably with those obtained by the methods described in Examples I and II.

EXAMPLE IV Preparation of potassium glucuronate in the presence of methanol :Botassium.glucur.onate was prepared by rapid METHANOL-WATER RATIO IN OB POTASSIUM GLUCURO- TABLE I.-VARYING CRYSTALLIZATION NATE DIHYDRATE [18.5 g. glucuronclactone 100 ml. of l N KOH at 70 F.]

Volume of Solvent (ml. Color of 57 PrYStamHF Solution held After 24 C t 1 (O Hours n) W 3 s 1 Methanol 1 Water D.) 1

75 Product thrown out as a syrup 60 24.1 7. 2 50 23. 6 2. 5 40 23. 6 1. 6 30 22. 8 0.96 20 21. 0 0. 60 10 90 1S. 2 0. 68 0 100 13. 4 0. 56

As used herein and hereinafter the term 0. D. means optical density (160 cm.) at 450 my minus optical density (160 cm.) at 650 my. This represents essentially a measurement of yellow and red color.

EXAMPLE V Preparation of potassium glucaronate with difierent potassium hydroxide concentrations In this experiment six samples of 6.6 g. of per cent potassium hydroxide were dissolved in the following amounts of water: (1) 25 ml., '(2) 25 ml., (3) 50 ml., (4) .75 ml., (5) 100 ml., (6) 200 ml. The first solution was cooled to 15 F. before addition of glucuronolactone, and the other five were cooled to 70 F. Then 18.5 g. of crystalline glucuronolactone was added instantaneously and stirred rapidly into each solution and the temperature rise and color formation noted in each case. Results appear in Table II. It is evident that raising alkali concentration above about 1.5 N causes increased color, and lowering the initial temperature of the alkali solution, thereby restricting the temperature rise during the reaction, only partially obviates this 55 TABLE II.EFFECT OF ALKALI CONCENTRATION IN THE PREPARATION OF POTASSIUM GLUGURONATE [18.5 g.glucuronolactone 6.6 g. 85% K011] Initial Final Water Used Order of Increasing In KOH ggff g 3 3? Color (diluted to (1111.) (0 Fa a R) 250 ml.)

25 15 3- dark yellow. 25 70 140 4-darl; red-brown. 50 70 106 2yellow. 75 70 96 l 100 70 90 1 very light yellow 200 70 80 1 EXAMPLE VI Preparation of potassium glacaronate with different stoichiometric ratios of giac'aronolactone and potassium hydroxide When exact stoichiometric equivalents of potassium hydroxide and glucuronolactone are combined the final pH value is above 7.0 and degradation of the product occurs in solution. The data in Table III are from an experiment in which different amounts of glucuronolactone were stirred rapidly into 70 m1. of 1.48 N potassium hydroxide followed by addition of 30 ml. methanol. Color of first crystals appears acceptable at a 6 per cent excess of glucuronolactone over alkali, where pH value dropped to below 8.0 after 20 minutes. Greater excesses of lactone do not increase the yield and are therefore uneconomical.

TABLE III.EFFECT or STOICHIOMETRIC EXCESS OF GLUCURONOLACTONE IN THE PREPARATION OF POTASSIUM GLUCURONATE pH Value of Reaction Color of 57 y 0 Stoichiometric Excess Mixture Solution of Lactone, Percent Crystals,

2min. 20min. 24hr.

EXAMPLE VII Preparation of potassium gluc'aronate with order, form, and rate of combination of reactants varied The effects of (1) order of combination of reactants, (2) addition of glucuronolactone in crystalline form or in solution, and (3) rate of addition of glucuronolactone, are shown in Table IV from the standpoint of color formation, which is directly related to yield and quality of product.

TABLE IV.EFFECT OF ORDER, FORM, AND RATE OF COMBINATION OF REACTANTS ON COLOR FOR- MATION 1N PREPARATION OF POTASSIUM GLU- CURONATE [Products diluted to 250 ml. for color determination] Order of Rate of Color, Addition Addition 0. D. Lactone (g.) 85% KOH (g.) H2O (ml) 18.5 (0.105 mole)..- 6.6 (0.100 mole). 10 34. 9 18.5 6.6 70 28. 6 6.6 11. 4 6.6 210 10. 4 6.6. 70 12. 2 6.6. 70 44. 3 6.6 70 16. 8 6.6 70 38. la 6.6 140 9. 8 6.6.... 140 48. 3 6.6.-.. 140 11.4 6.6 140 26. 0

It is clear from the data in Table IV that of the three variables discussed thus far, rate of combination of reactants is the most critical. High colors result both when glucuronolactone solution is slowly added to alkali and vice versa.

Order of combination of reactants is not critical when mixing is rapid, but comparing test to 12 it appears that when substantial time is required for mixing it is better to add the alkali solution to the lactone solution so that an excess of alkalinity prevails for the shortest possible time.

The data demonstrate that dry and dissolved glucuronolactone serve equally well for the reaction when mixing is rapid.

EXAMPLE VIII Preparation of sodium glacuronate with variable methanol-water ratio Sixty milliliter portions of 1.67 N sodium hydroxide solutions were made up with graded proportions of methanol and water and 18.5 g. glucuronolactone was stirred rapidly into each. Data appear in Table V.

TABLE V.-VARIABLE METHANOL-WATER RATIO IN CRYSTALLIZATION OF SODIUM GLUCURONATE MONOHYD RATE [18.5 g. glucuronolactone L 60 ml. 1.67 N sodium hydroxide at 70 F.]

57 Soln. Methanol Water g? Yield Or ystaIs, (ml.) (1111.) (72 hr.) (g.) oolgrl,

I 50 10 8. 8 hygroscopic precipitate. 40 9. 2 syrup 35 9.2 19.8--.." 19.7 30 8.8 19.0--- 0.52 25 8.3 18.3--- 0.32 20 7.8 13.4-.-" 0.24

These data show that too much methanol throws out syrup or inferior crystals and too little lowers first crop recovery. A 50-50 ratio is acceptable.

EXAMPLE IX Preparation of sodium glucaroaate with variable sodium hydroxide concentrations Five solutions of sodium hydroxide were made up in water to concentrations of 1.0, 1.25, 1.67, 2.50, and 5.00 N, and a 5 per cent stoichiometric excess of glucuronolactone was stirred rapidly into each. Color formation was excessive at the two highest alkali concentrations, but was substantially lower at and below 1.67 N. Except at the highest concentration of alkali the temperature never rose above 120 F. Since sodium glucuronate is considerably more soluble in aqueous 10 methanol than is potassium glucuronate, it is advantageous to use as high a concentration of sodium hydroxide as is consistent with tolerable color formation.

We claim:

1. The process of preparing alkali metal salts of glucuronic acid which comprises instantaneously and intimately contacting glucuronolactone and aqueous alkali metal hydroxide, and recovering the resultant salt; the temperature during the resultant reaction not exceeding about 130 F.; the ratio of glucuronolactone to alkali metal hydroxide being 1.0 mole to 0.9-1.0 mole and the concentration of alkali hydroxide being 0.5 to 2.0 moles per liter of hydroxide solution.

2. The process according to claim 1 wherein the resultant solution of the salt of glucuronic acid is decolorized with activated carbon at a temperature not exceeding about 130 F., the carbon removed and thereafter sufiicient methanol added to the clarified liquor to produce a concentration of methanol representing 20 to per cent by volume of the liquor, and the resultant crystalline salt of glucuronic acid recovered.

3. The process of preparing potassium glucuronate dihydrate which comprises instantaneously and intimately contacting glucuronolactone and potassium hydroxide in the molar ratio of 1.0 to 0.9-1.0, the concentration of potassium hydroxide being 1 N, the temperature of the resultant reaction being in the range of about to F., decolorizing the resultant solution with activated carbon at a temperature not exceeding F., removing the carbon, adding 20 to 40 per cent on a volume basis of methanol, and recovering the resultant crystalline potassium glucuronate dihydrate.

4. The process of preparing sodium glucuronate monohydrate which comprises instantaneously and intimately contacting glucuronolactone and sodium hydroxide in the molar ratio of 1.0 to 0.9-1.0, the concentration of sodium hydroxide being 1.7 N, the temperature during the resultant reaction being about 70 to 90 F., decolorizlng the resultant solution with activated carbon, removing the carbon, adding 35 to 55 on a volume basis of methanol, and recovering the resultant crystalline sodium glucuronate monohydrate.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,520,255 Peterman Aug. 29, 1950 2,562,200 Mehltretter July 31, 1951 2,583,534 Mast Jan. 29, 1952 

1. THE PROCESS OF PREPARING ALKALI METAL SALTS OF GLUCURONIC ACID WHICH COMPRISES INSTANTANEOUSLY AND INTIMATELY CONTACTING GLUCURONOLACTONE AND AQUEOUS ALKALI METAL HYDROXIDE, AND RECOVERING THE RESULTANT SALT; THE TEMPERATURE DURING THE RESULTANT REACTION NOT EXCEEDING ABOUT 130* F.; THE RATIO OF GLUCURONOLACTONE TO ALKALI METAL HYDROXIDE BEING 1.0 MOLE TO 0.9-1.0 MOLE AND THE CONCENTRATION OF ALKALI HYDROXIDE BEING 0.5 TO 2.0 MOLES PER LITER OF HYDROXIDE SOLUTION. 